Lignin-Based Fabrication of Co@C CoreâShell Nanoparticles as

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Lignin Based Fabrication of Co@C Core-Shell Nanoparticles as Efficient Catalyst for Selective Fischer-Tropsch Synthesis of C5+ Compounds Hengfei Qin, Shifei Kang, Yangang Wang, Huan Liu, Zhijiang Ni, Yongkui Huang, Yaguang Li, and Xi Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01269 • Publication Date (Web): 06 Jan 2016 Downloaded from http://pubs.acs.org on January 9, 2016

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ACS Sustainable Chemistry & Engineering

Lignin Based Fabrication of Co@C Core-Shell Nanoparticles as Efficient Catalyst for Selective Fischer-Tropsch Synthesis of C5+ Compounds Hengfei Qin a, Shifei Kang b, Yangang Wang b*, Huan Liu a, Zhijiang Ni a, Yongkui Huang a, Yaguang Lib, Xi Li a* a

Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China b

School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai 200093, China

*Corresponding author. Tel/Fax: +86 21 65642789. E-mail address: [email protected] (X. Li) 1

ACS Paragon Plus Environment


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Abstract: Novel Co@C core-shell nanoparticles were prepared by a straightforward low-temperature carbonization process. The industrially-available lignin was used as a low-cost bio-renewable carbon source for the first time. The products were characterized

by X-ray

diffraction,

energy-dispersive

X-ray

spectrometry,

transmission electron microscopy, nitrogen adsorption-desorption and Raman spectrum. The results showed that the synthesized Co@C catalysts had a well-defined core-shell structure with a moderate degree of graphitization, and the metal Co nanoparticles with the sizes of 20-150 nm were wrapped by several layers of graphitic carbon. This unique core-shell structure is useful in Fischer-Tropsch reaction since it can provide high adsorption space and the graphite carbon layer defects are beneficial for H2 dissociative adsorption. Furthermore, the shell of graphitic carbon layers could restrict the aggregation of the cobalt nanoparticles during the activation and reaction processes. Fischer-Tropsch synthesis results exhibited the Co@C core-shell catalysts had a high catalytic performance with the highest C5+ selectivity up to 56.8%, which is much higher compared with the traditional Co/AC catalyst (46.2%). Keywords: Carbon materials; Core-shell structure; Nanoparticles; Fischer-Tropsch synthesis; Introduction Fischer-Tropsch synthesis (FTS) is an effective method for production of liquid fuel and chemicals from syngas (CO+H2). Liquid fuels produced by FTS do not contain nitrogen, sulfur, or aromatic compounds. Therefore, FTS has triggered many studies attention as an attractive production method for liquid fuels, and their studies about the influence of different active components, catalyst synthesis methods, support species, introduction of promoters, etc. have been investigated.1-5 Recently 2


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nanocatalysts with a core-shell structure have received great interest in different processes because of their superior catalytic activity.6-8 Nevertheless, in these cases few studies on FTS were reported.9-11 In one study a novel catalyst (Co@Ru nanoparticle over Al2O3), was prepared and compared with Co/Al2O3, it substantially improved the production of C2-C4 from syngas in FTS.9 Moreover, the core-shell structure Fe@HZSM-5 can improve catalytic performance in gasoline production.10 The FexOy@C Spheres prepared from hydrothermal synthesis showed higher CO conversion and C5+ selectivity in FTS.11 Erokhin et al. synthesized Fe@C and Ni@C nanostructures and founded that the graphite carbon shell was able to activate H2.12 Graphite carbon can be produced by different ways including direct carbonization on the metal surfaces.13 Some evidences prove that metal matrix can influence the physical and chemical properties of graphene-like carbon layer.12,14 Therefore, we consider that metal nanoparticles encapsulated by the graphite carbon to form core-shell metal-carbon nanostructure may provide unusual catalytic properties in FTS. Up to now, catalyst materials with core-shell graphite frameworks, various synthesis approaches, such as arc discharge,15,16 chemical vapor deposition (CVD),17 template approach,18 pyrolysis of organic precursors,19 and sol-gel method20 had been developed to fabricate core-shell catalyst materials. However, most of these synthesis methods were used fossil fuels as carbon sources, such as ethanol, acetylene and pyridine, which may not be sufficient in near future. As a result, it is crucial to switch to the cheap and renewable carbon sources. Lignin is a cheap, carbon-rich bio-renewable resource and the second most abundant biopolymer in nature after cellulose in the nature,21 which is usually acted as a waste to discharge from the paper factory. However, lignin is composed of repeated 3



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units of the phenylpropane: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol,22 which makes it highly polar with a large amount of hydroxyl group (-OH). As is well-known, phenol is a widely-used carbon precursor in synthesis of carbon nanomaterials due to its plentiful hydroxyl groups.23 Because of the structural similarity of lignin with phenol (-OH), lignin can be a promising natural alternative for precursor in synthesis of carbon materials, and for preparation of cheaper and less-toxic products. Thus, lignin has been investigated as a precursor for the production of carbon materials because of its low cost, bio-renewablility, nontoxicity and -OH groups structure. Therefore, lignin is an important a carbon source to prepare carbon materials. Lignin has already been used to prepare carbon fibers,24 activated carbons,25carbon nanotubes,26 and porous carbon materials.27 To the best of our knowledge, however, Co@C core-shell catalysts prepared with lignin as a carbon source have not yet been reported. Herein, we report a novel straightforward method for the synthesis of Co@C core-shell catalysts by a low temperature carbonization process using lignin as the carbon source and cobalt nitrate as a Co precursor. The structural properties of the samples as-obtained were characterized by nitrogen adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), Temperature-programmed reduction (H2-TPR), transmission electron microscopy (TEM) and Raman spectroscopy.

Furthermore,

the

Fischer-Tropsch

Synthesis

(FTS)

reaction

performances of the Co@C core-shell catalysts in terms of CO conversion, CH4 selectivity, C2-C4 selectivity and long-chain hydrocarbons selectivity were investigated and compared with the Co/AC catalyst.

4


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Experimental Preparation of lignin based oligomer precursors. Alkali lignin was purchased from Nanjing Forestry University. All the chemicals were used as received without further purification. The lignin-based oligomer precursors were prepared by using lignin and formaldehyde in a base-catalyzed process according to our own method.13 For a typical preparation, 7.0 g of lignin was added into 10.0 g of 5 wt% NaOH water solution under stirring at 40 oC. After 10 min, 8.1 g of formalin solution (37wt% formalin) was added dropwise, and the mixture was reacted at 70 oC for another 60 min. After the mixture cooled down to 30 oC, the precursors were redissolved in water (20wt% water solution). Synthesis of Co@C catalysts. The Co@C nano-structure catalysts were synthesized by low-temperature carbonization reaction. Briefly, 1.0 g Co(NO3)2·6H2O (7.5wt% water solution) was added into 30 g of lignin-based oligomer precursors (20wt% water solution). After stirring for 60 min, the dark-brown oligomer was polymerized at 100 oC for 12 h, then peeled off, and carbonized at 400, 600 or 800 oC for 3 h with a ramping rate of 3 oC /min under Argon flow. The produced sample was respectively denoted by the carbonize temperature as Co@C-400, Co@C-600 and Co@C-800, respectively. The schematic illustration of the preparation method of Co@C catalyst is shown in Fig. 1. For comparison, activated carbon (AC) supported cobalt catalyst (Co/AC) with the same weight percentage of Co content was synthesized as a previous reports,28 The Co/AC based catalyst was prepared by the incipient wetness impregnation of AC (Hebei Hua Jing Activated Carbon Co., Ltd) with an aqueous solution of cobalt nitrate. The wet sample was dried at 80 oC for 10 h in air and ﬁnally calcined at 600 oC for 3 h under pure Ar. The sample was denoted as Co/AC-600. 5



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Measurements and Characterizations. The structural properties of the obtained Co@C core-shell nanocage materials were identified using a Rigaku D/Max2rB-II X-ray diffractmeter (XRD, Cu Kα radiation, λ=1.5406 Å). The tube voltage was 40 kV, and the current was 100 mA. The 2θ angles were scanned from 10 to 80° at 8° min−1. Transmission electron microscope (TEM) images were taken on a JEOL JEM-2010 electron microscope with an acceleration voltage of 200 kV. The Raman spectrum was recorded on a Horiba XploRA Raman microscope using a 532 nm argon ion laser. The laser beam was focused into an area of 100 µm in diameter on sample surface, with a power of 5mW. Nitrogen sorption isotherms were measured at -196 oC on a Micromeritics ASAP 2000 apparatus. The speciﬁc surface area of the samples was calculated using the multipoint Brunauer-Emmett-Teller (BET) method within the relative pressure range of 0.05-0.35. Temperature-programmed reduction (TPR) experiments were acquired on a home-made apparatus equipped with a thermal conductivity detector (TCD). The catalyst sample of 0.15 g was reduced from room temperature to 800 oC in 10% H2/Ar with a heating ramp of 10 oC /min. X-ray photoelectron spectrum (XPS; Perkin Elmer PHI5000C) were recorded using a Mg Kα radiation as the excitation source (1253.6 eV). The sample was pressed into a self supported disc, and degassed in the pretreatment chamber in vacuum at room temperature for 4 h. All binding energy (BE) values were referenced to the C1s peak of contaminant carbon at 284.6 eV. FTS products were analyzed online with two gas chromatographs. A TDX-01 packed column connected to the TCD was used to analyze H2, N2, CO, CH4, and CO2, while hydrocarbons (C1-C30) were analyzed with a PONA capillary column connected to a flame ionization detector (FID). Fischer-Tropsch synthesis. Fischer-Tropsch synthesis reaction was performed in a tubular fix-bed reactor (i.d.=10 mm) at 270 oC, total pressure 2 MPa, and H2/CO 6


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ratio = 2. About 0.5 g of catalyst was loaded into the isothermal region of the reactor for all the reaction tests. The remaining volume of the reactor tube was filled with quartz granules in a size range of 40-60 mesh. Then the syngas H2/CO/N2 (64/32/4) was passed over the catalyst bed at a flow rate of 30 mL/min (GHSV =3.6 L/hr/gcat) and the pressure was increased gradually to 2 MPa and the temperature was raised to 270 oC. Nitrogen was used as an internal standard in order to ensure an accurate mass balance. Once the reaction temperature of 270 oC was achieved, the reaction was allowed to proceed during a period for 8 h. The FTS products were analyzed on-line with two gas chromatographs. A TDX-01 packed column connected to the TCD was used to analyze H2, N2, CO, CH4, and CO2, while hydrocarbons (C1-C30) were analyzed with a PONA capillary column connected to a flame ionization detector (FID). The hydrocarbon selectivities were calculated on the carbon basis. All the Fischer-Tropsch synthesis reaction tests of the as prepared catalysts were performed twice in order to reduce the relative errors of catalytic activity and product selectivity within 10%. Results and discussion Fig. 2a compares the X-ray diffraction (XRD) patterns of catalysts (Co@C-400, Co@C-600 and Co@C-800) as-prepared at different preparation stages. Three intense peaks located at around 44.2º, 51.5º and 75.8º can be respectively indexed as (111), (200) and (220) diffraction of metal cobalt according to the JCPDS Card Number 15-0806. Compared with Co@C-600 and Co@C-800, the peaks of the metal cobalt in Co@-400 sample formed at the low calcination temperature are much weaker, which is attributed to its lower crystallinity and smaller Co particles size. As reported, 28-30 7



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the cobalt phase is presented in XRD pattern as Co3O4 form. In this work, due to the use of reducers (lignin-based oligomer and active carbon), metal cobalt was formed after calcinations process. In addition, the broad diffraction peaks located at 2θ around 26.1º can be attributed to the graphitic carbon,13, 15 and the intensity of this graphitic peak becomes stronger with the increase of calcination temperature. To further investigate the graphitization state of Co@C catalysts, Raman spectroscopy test was carried out to identify the vibration of carbon species and the defects in graphite carbon layers. As shown in Fig. 2b, the Co@C-400 catalyst displayed a D band at 1335 cm-1, a G band at 1574 cm-1 and a 2D band at 2693 cm-1, whereas the Co@C-600 and Co@C-800 catalysts both showed two bands around 1364 cm-1 (D band) and 1597 cm-1 (G band). The D band is attributed to an A1g vibration mode of carbon atoms with a double-resonance process in plane terminations of disordered graphite.31 The G band corresponds to the E2g mode of graphitic carbon and assigned to the vibration of sp2 hybridized carbon atoms in the graphite layer. G-band in amorphous carbons typically is situated at 1580 cm-1, but in the Co@C catalysts under study, the broadened peak is positioned at 1597 cm-1.32, 33 There is also a new peak at 1615 cm-1, called D′, which occurs via an in-travalley double-resonance process in graphite carbon layer containing defects.32 Also both spectra show a 2D peak at 2712 cm-1, which is an overtone of D peak oscillations. The D+D′ peak at 2981 cm-1 confirms the presence of D′ peak and corresponds to combination mode of D and D′ peaks oscillations according to.12, 32 Generally, the relative integrated intensity of these two bands (ID/IG, which was calculated according to the area of corresponding peak) can be used to reflect the disorder degree of functionalized groups and defects or the degree of graphitization, which is calculated to be 1.53, 1.36 and 1.16 for Co@C-400, Co@C-600 and Co@C-800 respectively, 8


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suggesting an enhancement of graphitization degree with increasing of pyrolysis temperature. On the other hand, a greater ID/IG ratio indicates a higher the disorder degree of graphite. Thus according to Raman scattering spectra, carbon shells in Co@C consist of defective graphite carbon layers.12 The results are similar with a previous work.34 H2-TPR experiments were performed to evaluate the reducibility of the catalysts. All the Co@C catalysts show two reduction peaks located at 378 and 467 °C, respectively (Fig. 3a). The first peak can be attributed to reduction of CoO to metallic cobalt,9, 30 and the latter peak located about 450-550 oC is attributed to the catalytic decomposition of surface carbonaceous substrate,35-37 which is also observed in the lingin precursor-600. Catalyst Co/AC shows two different hydrogen consumption peaks at 335 and 632 oC, can be assigned to the following consecutive reduction of Co3O4 to CoO and CoO to Co, respectively. The Co@C catalyst do not show any reduction peak above 600 °C indicates that no hardly reducible compounds exist on the catalyst surface due to the much weaker interaction between graphite carbon support and cobalt species compared with the traditional oxide (Al2O3, SiO2 or TiO2) supported catalyst.38 To further investigate the chemical valence of Co@C catalysts, X-ray photoelectron spectroscopy (XPS) test was carried out to define the chemical state and cobalt species over the catalyst surfaces. Fig. 3b shows the Co 2p spectra of the Co@C catalysts before and after reduction. For the Co@C-400, Co@C-600 and Co@C-800 samples, the Co 2p3/2 BE is 781.6 eV, and the BE difference between the 2p1/2 and 2p3/2 levels is 15.7 eV, which can be ascribed to CoO.36, 39, 40 After reduction, the Co@C-600a Co 2p3/2 peaks with BEs of 778.4eV and the BE difference between the 2p1/2 and 2p3/2 levels is 15.1 eV, which can be assigned to metallic Co.41

9



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The size and morphology of the samples were characterized by TEM. A carbon-encapsulated core-shell structure is clearly showed in Fig 4c, 4e, where the graphitic carbon layers with well-ordered arrangement are closely compacted to form the shell structure and surround the dark cobalt nanoparticles. According to the HRTEM images (Fig. 4d, 4f), the Co@C particles are composed of metal core covered with several graphitic carbon layers. However, the core-shell structure of Co@C-400 is not observed (Fig. 4a, 4b), probably because graphite structure was not easy to form at lower calcination temperature. As showed in Fig. 4c, the Co@C-600 core-shell catalysts have an outer diameter of ~60 nm and a shell thickness of ~16 nm. The HRTEM image of an isolated 38 nm Co@C-600 catalyst (Fig. 4d) reveals the core-shell structure. The lattice spacing in the core and in the shell were 0.204 and 0.34 nm, respectively, corresponding to the (111) plane of cobalt and the (002) plane of graphite. Besides, some dislocations and disorder parts in graphite layers can be detected from HRTEM images in Fig. 4d and 4f, which should be related to the lattice defects of carbon layers. 42, 43 The TEM image in Fig. 4 also indicates that the situation is different for Co@C-600 and Co@C-800: the cobalt core of each particle is fully covered by graphitic carbon shell. The graphitic carbon shell is not uniform and its thickness is from 6-8 to 26-30 layers, while the core size of cobalt nanopaticles is also different and its value ranges from 20-30 to 80-150 nm. This phenomenon could be attributed to (i) different rates of graphitic carbon shell formation on edges of cobalt crystal during the synthesis or (ii) migration of cobalt atoms through the graphitic carbon shell to the surface under the influence of calcination temperature.12 As shown in Fig. 5a the N2 adsorption-desorption isotherms of all the Co@C catalysts show a strong uptake at relative pressure (P/P0) of 0.9-0.99 suggesting the 10


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macroporous characteristics. In addition, another hysteresis loop of Co@C-600 and Co@C-800 catalysts is located at P/P0 between 0.4 and 0.7, indicating that both samples have mesoporous structures.44,45 The BET surface areas of Co@C-400, Co@C-600 and Co@C-800 catalysts are 12.5, 184.3, 236.3m2·g-1, respectively and their pore volumes are 0.26, 0.39, 0.47cm3·g-1, respectively. Fig.5b shows the thermos-gravimetric analysis (TGA) result of the Co@C catalysts under the air atmosphere. The weight loss below 200 °C was caused by the desorption of the physisorbed water, while a significant weight losses occurred in the range of 370-600°C can be assigned to the combustion of graphitic carbon. It can be found that the pyrolysis temperature of carbon is increase with the increasing of its graphitization degree, which has confirmed by many previous literatures.46, 47 Table 1 gives the results of FT synthesis from the different catalysts, including the catalytic activity and product selectivity. The main products obtained were CH4, CO2, C2-C4 and long-chain hydrocarbons (C5+). These latter products included liquid hydrocarbons and waxes. All the catalytic tests were performed twice, and the relative errors obtained in all cases were less than 10%. Clearly, the cobalt nanoparticle with core-shell structure in the catalysts resulted in higher reaction rate. Thus, the CO catalytic activities were much higher for samples Co@C-600 and Co@C-800 than those obtained with other catalysts. 14.8Co@C-400 and 15.1Co/AC-600 gave a CO conversion of 20.1% and 41.1%, a typical ASF product distribution with selectivities to CH4 and C5+ of 21.2%, 21.6% (14.8Co@C-400) and 33.7%, 46.2% (15.1Co/AC-600). However, the encapsulation of graphitic carbon to cobalt catalyst (e.g. Co@C-600), had a higher CO conversion (78.6%), lower selectivity to CH4 (13.6%) and higher selectivity to C5+ (56.8%). Furthermore, Co@C-600a and Co@C-600d have similar results, which indicate that metal cobalt is major active 11



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phase of Co@C-600 in FTS. Meanwhile, it can be seen that the CH4 and CO2 production are relative higher in all samples, and the values are larger than that of previous cobalt-based catalysts,48,49 which due to the relatively higher reaction temperature we adopted since the selectivity of CH4 and CO2 would increases with the increasing of the reaction temperature.36,50,51 The Co@C catalysts exhibit a higher catalytic activity, which is attributed to the triple synergy effect based on the special core-shell structure of Co@C catalysts. Firstly, the core-shell structure of Co@C nanoparticles with graphite frameworks can promote the electronic conductivity between the Co metal and the CO molecules52 and enhance the catalytic activity of Co@C catalysts. Secondly, the defective graphite carbon layers would facilitate the electron transfer between the cobalt species and CO molecules,52,53 which makes the cobalt surface easier to absorb more carbon monoxide,12 and is beneficial to the formation of C5+. Furthermore, the shell of graphite carbon layers restricted the aggregation of the cobalt nanoparticles during the activation and reaction processes,43 which ensured the activity of cobalt nanoparticles in the core. This confinement effect may extend the contact time between cobalt active sites and CO molecules, thus improving selectivity to C5+. Fig. 6 shows the CO conversion of the Co@C catalysts and Co/AC catalyst as a function of time-on-stream. The CO conversions on all the catalysts, except Co@C-600d, increased with time and steadyed off within 8 h, followed by a slow drop in activity. The decrease in CO conversion may be attributed to the cobalt carbides of nanosized cobalt crystallites,54 which could be confirmed by the XRD (see Fig. 7a). Meanwhile, we can see that the Co/AC catalyst presents lower CO conversion than the Co@C catalysts (Co@C-600 and Co@C-800). The CO 12


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conversion of Co@C-600 catalyst was 89.2% initially and then decreased to 77.9% after 86 h on stream, and stabilized afterwards. For comparison, CO conversion on the Co/AC-600 catalyst dropped much faster (from 56.5 to 32.8%) within 70 h of testing. The possible reason is that the Co@C catalysts containing defective graphite carbon layers are beneficial for electronic conductivity between the Co metal and the CO molecules and would restrict the aggregation of cobalt nanoparticles during the activation and reaction processes, which provides extremely high catalytic activity and selectivity to C5+. Fig. 7a shows that the used catalysts exhibit obvious peaks at around 44.2º, 51.5º and 75.8º resulting from metal cobalt (JCPDS Card No. 15-0806). In addition, peaks corresponding to cobalt carbide can be clearly observed for the Co@C catalysts. The diffraction peaks centered at 2θ of about 37.5º, 43.1º and 45.9º appeared, which are attributed to (101), (211) and (021) diffraction planes of the cobalt carbides (Co2C, JCPDS Card No. 50-1371). The broad diffraction peak at 2θ = 26.1º of the used catalyst is relatively lower than the fresh catalysts (as-prepared), which indicates more defects in graphite carbon layers. Fig. 7b gives the relative intensity ratio of D band and G band, ID/IG, is ~1.52, indicating lower graphitization degree and higher in-plane defects in Co@C-600 (used) catalyst, the result is consistent with XRD pattern. The TEM images and EDX spectra of the Co@C-600 catalysts after 100 h of FTS are shown in Fig. 8. TEM images of the used catalysts show that the core-shell structure is similar to the fresh catalysts after running for 100h (Fig. 8b). No obvious agglomerations of cobalt nanoparticles are observed in the TEM images. Besides, many dislocation defects are found in graphite layers (Fig. 8c), which further confirms the conclusions of XRD pattern and Raman analysis (Fig. 7). The Co@C catalysts exhibit a higher catalytic activity and selectivity to C5+ hydrocarbons, which is 13



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attributed to the core-shell structure with graphite frameworks. The shell of graphite carbon layers restricted the aggregation of the cobalt nanoparticles during the activation and reaction processes. The shell of graphite carbon layers containing many defects, which is beneficial for electronic conductivity between the Co metal and the CO molecules and formation of C5+ hydrocarbons. The structure of the used Co@C-600 catalyst was studied using energy-dispersive X-ray spectrometry (EDX) and the result was shown in Fig. 8d. Clearly, the cobalt particles appear at 0.775 and 6.9 keV. However, the peak at 6.9 keV can also be attributed to a new phase of cobalt carbides,55 which corresponded well to the results of XRD pattern (Fig. 7a). Conclusions In summary, the novel Co@C core-shell catalysts were synthesized by a direct carbonization method at relatively low carbonization temperature of 600 oC. The industrially-available, renewable lignin was used as a carbon sources. XRD, EDX, TEM, XPS, H2-TPR and Raman results all consistently reveal the Co@C catalysts have a core-shell catalyst structure with relative high surface area and mesostructure. The results of FT synthesis indicate that the Co@C core-shell catalysts establish an excellent FT catalytic performance, while the Co@C-600 catalyst show the highest catalytic performances, especially the pretty high C5+ selectivity of 56.8%. The remarkable high activity of the Co@C catalysts owe to the unique core-shell structure, which lead to reduced H2 dissociative adsorption energy, enhanced electronic conductivity between the Co metal and the CO molecules, and suppressed aggregation of the cobalt nanoparticles. Author Information Corresponding Authors *E-mail: [email protected] (Y.G. Wang) 14


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*E-mail: [email protected] (X. Li) Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.21103024, No.61171008 and No.51502172), the company of Yancheng Huanbo Energy Technology. We also would like to thank Qiming Liu Professor of Nanjing Forestry University provide lignin and Xia Bai, Chaochuang Yin for experimental technique support. References (1) Sun, Z.; Sun, B.; Qiao, M.; Wei, J.; Yue, Q.; Wang, C.; Deng, Y.; Kaliaguine, S.; Zhao,

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(52) Fan, Z. L.; Chen, W.; Pan, X. L.; Bao, X. H., Catalytic conversion of syngas into C2 oxygenates over Rh-based catalysts-Effect of carbon supports. Catalysis Today 2009, 147, 86-93. (53) Ning, X. M.; Yu, H.; Peng, F.; Wang, H. J., Pt nanoparticles interacting with graphitic nitrogen of N-doped carbon nanotubes: Effect of electronic properties on activity for aerobic oxidation of glycerol and electro-oxidation of CO. J Catal 2015, 325, 136-144. (54) Pei, Y. P.; Ding, Y. J.; Zhu, H. J.; Du, H., One-step production of C1-C18 alcohols via Fischer-Tropsch reaction over activated carbon-supported cobalt catalysts: Promotional effect of modification by SiO2. Chinese J Catal 2015, 36, 355-361. (55) Tavasoli, A.; Trepanier, M.; Abbaslou, R. M. M.; Dalai, A. K.; Abatzoglou, N., Fischer-Tropsch synthesis on mono- and bimetallic Co and Fe catalysts supported on carbon nanotubes. Fuel Process Technol 2009, 90, 1486-1494.

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Figure and table Captions Fig. 1 The scheme of Co@C catalysts formation during synthesis. Fig. 2 (a) XRD patterns and (b) Raman spectra of Co@C catalysts. Fig. 3 (a) H2-TPR profiles of Co@C catalysts and (b) XPS spectra of Co@C catalysts before and after reduction. (a denote as after reduction, reduction conditions: 5% H2/Ar, 50 mL/min, 400 °C, 16 h) Fig. 4 TEM and HRTEM images of Co@C catalysts (a) and (b) Co@C-400, (c) and (d) Co@C-600, (e) and (f) Co@C-800. Fig. 5 (a) N2 adsorption-desorption isotherms of the Co@C catalysts and (b) TG curve of the Co@C catalysts with different temperature. Fig. 6 Conversions of CO with time on stream for Co@C and Co/AC catalysts. (d Reduced in 5% H2/Ar (50 mL/min) for 16 h at 400 °C before F-T reaction) Fig. 7 (a) XRD patterns and (b) Raman spectra of fresh and used Co@C-600 catalyst. Fig. 8 (a), (b) TEM, (c) HRTEM image and (d) EDX spectra of Co@C-600 used catalysts. Table 1 Comparison of the Fischer-Tropsch Synthesis Catalytic Performances between the Co@C and Co/AC catalysts

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

solvent evaporation lignin-cobalt composite cobaltous nitrate lignin oligomer

carbonization

Co@C nanoparticles

graphitic carbon cobalt

Fig. 1 The scheme of Co@C catalysts formation during synthesis.

24


Page 24 of 33

Page 25 of 33

ο Co

(a) ο ο

Co/AC-600

Intensity (a.u.)

♦ graphitic

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ο

♦

Co@C-800 Co@C-600

D G, D' 1364 1597 1615

Co@C-800

Co@C-600

Co@C-400

20

30

40 50 60 2θ (degree)

70

(b) 2D D+D' 2712 2981

Co@C-400 80

1000

1500 2000 2500 -1 Raman shift (cm )

Fig. 2 (a) XRD patterns and (b) Raman spectra of Co@C catalysts.

25


3000


Lignin precursor-600

Co 2p 1/2

(a)

Co/AC

Co 2p 3/2

(b)

PE Intensity (a.u.)

Co@ C-800

TCD signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

Co@ C-600 Co@C-400

Co@C-600

a

Co@C-800 Co@C-600 Co@C-400

100

200

300

400

500

600

700

o

800

820

Temperature ( C)

810

800

790

780

770

760

Binding Energy (eV)

Fig. 3 (a) H2-TPR profiles of Co@C catalysts and (b) XPS spectra of Co@C catalysts before and after reduction. (a denotes “after reduction”, reduction conditions: 5% H2/Ar, 50 mL/min, 400 °C, 16 h)

26


Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4 TEM and HRTEM images of Co@C catalysts (a) and (b) Co@C-400, (c) and (d) Co@C-600, (e) and (f) Co@C-800.

27



100

400

3

250

Co@C-800 Co@C-600 Co@C-400

weight loss (%)

300

(b)

(a)

350

Vads (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

200 150 100

80 60

Co@C-800 Co@C-600

40

Co@C-400

20

50

0

0

0.0

0.2

0.4

0.6

0.8

1.0

0

P/P0

200

400

o

600

800

Temperature ( C)

Fig. 5 (a) N2 adsorption-desorption isotherms of the Co@C catalysts and (b) TG curve of the Co@C catalysts with different temperature.

28


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 CO conversion (%)

Page 29 of 33

Co@C-600 d Co@C-600

80

Co@C-800

60 Co/AC-600

40

Co@C-400

20 0

0

20 40 60 Time on stream (h)

80

100

Fig. 6 Conversions of CO with time on stream for Co@C and Co/AC catalysts. (d Reduced in 5% H2/Ar (50 mL/min) for 16 h at 400 °C before F-T reaction)

29



ο Co ο

(a)

♦ graphitic

Intensity (a.u.)

♦

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

• C o2C ο

C o@ C -600 fresh

♦

ο

• • •

ο

C o@ C -600 used

40

60

G, D' 1603

(b) 2D 2715

Co@ C-600 fresh

D+D' 2984

Co@C-600 used

ο

20

D 1363

Page 30 of 33

80

800

1200 1600 2000 2400 2800 3200 -1

Raman shift (cm )

2 θ (degree)

Fig. 7 (a) XRD patterns and (b) Raman spectra of fresh and used Co@C-600 catalyst.

30


Page 31 of 33

2.5

C

(d)

2.0

KCnt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5 1.0 0.5

Co

o

Co

Cu

0.0 0

1

2

3

4

5

6

7

E (keV)

Fig. 8 (a), (b) TEM, (c) HRTEM image and (d) EDX spectra of Co@C-600 used catalyst.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

Table 1 Comparison of the Fischer-Tropsch Synthesis Catalytic Performances between the Co@C and Co/AC catalysts. Catalyst

Selectivity b (%)

CO conv. (%)

CH4

C2-C4

C5+

CO2 selectivity (%)

C=/Cn c

14.8eCo@C-400a

20.1

21.2

45.1

33.7

17.1

1.7

14.8eCo@C-600a

78.6

13.6

29.6

56.8

15.3

2.7

14.8eCo@C-800a

60.3

15.3

31.8

52.9

18.9

1.1

14.8eCo@C-600d

80.6

12.3

30.4

57.3

16.7

2.9

15.1eCo/AC-600a

41.1

21.6

32.2

46.2

19.8

0.5

a

Reaction condition: T = 270 °C, P = 2MPa, H2/CO = 2, GHSV =3.6 L/hr/gcat, TOS = 36 h.

b

Hydrocarbon selectivity was normalized with the exception of CO2. c C=/Cn is the molar ratio of olefin to paraffin with C2-4.

d

Reduced in 5% H2/Ar (50 mL/min) for 16 h at 400 °C before F-T

reaction. e The content of cobalt in catalyst was determined by ICP.

32


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For Table of Contents Use Only

Lignin Based Fabrication of Co@C Core-Shell Nanoparticles as Efficient Catalyst for Selective Fischer-Tropsch Synthesis of C5+ Compounds Hengfei Qin a, Shifei Kang b, Yangang Wang b*, Huan Liu a, Zhijiang Ni a, Yongkui Huang a, Yaguang Lib, Xi Li a* a

Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China b

School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai 200093, China

Bio-renewable lignin was used as carbon source to synthesize graphitic carbon-encapsulated Co@C nanoparticles, which exhibited excellent catalytic performance in Fischer-Tropsch synthesis.

33


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